IPC-CM-770D-1996 - 第46页

January

1996

IPC-CM-770

Heat sinks may be required and should be properly secured

to the board

so

that they do not stress the subsequent sol-

der joints. A resilient spacer may be used as shown in

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“Resilient spacer

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IPC-I-O0203

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Figure 10-5 Resilient Spacer to Heat Sink Frame

Figure

10-5.

The lead-to-body seals of mounted devices should be

undamaged. Body chipouts that extend to or into the glass

seal and chipouts that expose a normally encased area of a

lead, are unacceptable. Hairline cracks in either the seal or

the body are not acceptable.

10.3 Surface Mounting

For whatever reason the deci-

sion is made to use through-hole components in combina-

tion with surface-mount components, there must be a will-

ingness to compromise some of the advantages of a totally

surface mount assembly. Modified through-hole compo-

nents do not provide the space savings that components

intended for surface mounting provide. However lead

modification may be the only reasonable approach to sur-

face mounting, if the required components are not available

in surface mounted packages.

10.3.1 Component Preparation

One method for mount-

ing both through-hole and surface mounted components on

the same board is to modify the leads

so

that they can be

surface mounted and soldered.

10.3.1.1 DIPS

Where only a few DIPs are involved on an

otherwise surface mounted assembly, the leads may be

formed to resemble a SOIC package. (See Figure

10-6.)

For production, machines are available which cut and form

IC leads into a “gullwing”-type configuration. (See Figure

10-6.)

Another method of modifying DIPs for surface mounting is

the “Butt” mounting technique. This involves simply cut-

ting the DIP leads to a shorter length and placing the

device on a land pattern to be soldered along with the other

surface mounted devices.

10.3.1.2 SIPS

SIPs lead forming is shown in Figure

10-7.

Take care to assure the coplanarity of the reformed

leads, especially if the assembly is to be vapor phase

reflow soldered. Leads may be carefully formed by hand

for low volume or prototypes. For production, machines

are available which cut and form IC leads into SOIC type

configuration.

Dip, thru-hole mount

Dip, modified

“SOIC”

type mount

(gull-wing lead)

Dip, modified

“BUTT”

mount

(“I”

lead)

IPC-I-O0206

Figure 10-6 Modifying DIP for Surface Mounting

10.3.2 Land Patterns

Land patterns for DIPs and

SIPs

that are surface mounted depend on the lead shape of the

parts. The gullwing lead shape uses the same land pattern

concepts for SOICs described in Section

9.

When using

DIPs with the “1”-lead, the land pattern should be suf-

ficient to provide a minimum of

0.25

mm on all sides of

the lead.

Figure 10-7 Gull-Wing Lead for SIP-type Component

10.3.3 Lead Configuration After Assembly

The lead

configuration for surface mounting for SIPs and DIPs

remain the same after assembly as before assembly. If

reflow soldering techniques are used, the leads must be in

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

contact with the solder paste or tin/lead plating being

reflowed. Proper handling is required

so

that components

do not move prior to the reflow soldering operation.

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IPC-I-O0270

Figure 10-8 Dual-in-line Package Gripping Tools

10.3.4 Mounted Component Configurations

The

mounting configuration for SIPS and DIPs are such that the

body of the part is parallel with the printed board to which

the part is attached.

10.4 Mixed Technology

SIPS and DIPS are normally

intended to be through-hole mounted. If parts are to be sur-

face mounted the requirements of Section

10.3

and Section

22

on surface mounting apply. In a mixed assembly, one

that contains both surface mounted and through-hole

mounted components, the decision of how to mount these

part types is determined during the design phase and is

usually based on the number of parts of a given type with

the intentions to reduce the number of assembly or process

steps. Land patterns must be available for the type of

attachment technique specified.

10.5 Manual Assembly

The sequence of equipment

mounting steps used for mixed assemblies is covered in

Section

25,

and should be such that no detrimental influ-

ence from any assembly operation is imposed on previ-

ously assembled components. Special fixtures or bending

techniques may be required to maintain component

placemendposition integrity prior to the solder process.

As

an alternative, partial assembly and soldering may be

required before a second set of electronic components are

mounted to the

packaging/interconnection

structure. Figure

10-8

shows part holders and dispensers, and dual-in-line

package gripping tool.

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IPC-I-O0208

Figure 10-9 DIP Clearances

10.6 Automated Assembly

Dual-Inline-package (DIP)

integrated circuits are among the most common electronic

devices to be processed. The obvious advantages of auto-

matically inserting DIPs are speed, volume, and accuracy.

In nearly all cases of printed board design where DIPs are

used, it is recommended that all DIPs be inserted in a

single orientation.

The component space requirements for DIP components

are similar to axial lead components in that the general

rules for axial lead component layout apply whenever they

are intermixed with DIPs on the same board. The DIP to

DIP clearances for the tooling fingers are shown in Figure

10-9.

In all cases of board design because the DIP is

located by the front leads, it is recommended that all DIPs

be laid out in a row or column fashion, starting at the front

or “operator” side of the board (refer to Figure

10-10).

The DIP leads should be uniform and not bent too far from

the specified positions. Machines do form and straighten

the leads within the machine, but leads bent too

far

out of

tolerance cannot be formed and will cause jams in the

machine.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

PROGRAM

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IPC-I-O0209

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Figure

10-10

DIP Layout in

Rows

and Columns

For automatic insertion, the DIP components should be

obtained from the supplier in slide magazines (see Figure

10-11)

for feeding into the insertion machines. Each slide

magazine can hold from 20 to

50

DIP components. The

capacity of the slide magazine will depend upon the length

of the DIP.

IPC-I-O0210

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Figure

10-11

DIP Slide Magazines

DIP modules may be inserted into sockets mounted on the

printed boards as well as directly into predrilled holes.

Wherever it is desired to insert into sockets rather than

directly into the circuit board, the cut and clinch mecha-

nism can be disabled. In the case of wire wrap sockets the

cut and clinch mechanism may be completely removed to

provide the necessary clearance for the socket leads.

10.7

Soldering

General soldering techniques applying to

all types of components are described in Section 27.

10.8

Cleaning

General information for cleaning apply-

ing to all types of components are described in Section 28.

10.9

Conformal Coating

General techiques for confor-

mal coating for all types of assemblies are described in

Section 29.

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RIBBON-LEAD COMPONENTS

Flatpacks and quad packs are similar to

“SO”

devices (see

Section 9), but with a number of important differences.

These devices are normally supplied by the vendors with

the leads extending straight out from the body on two

opposite sides, or from all four sides. Although this

requires the user to form the leads, this permits either sur-

face mounting or through-the-board mounting techniques.

(“SO”

device leads are pre-formed by the vendor for sur-

face mounting only.)

These devices are available in both hermetically sealed and

molded plastic styles. Hermetically sealed devices are

packaged in metal, ceramic, glass, or combinations of these

materials.

“SO”

devices are normally available only in

molded plastic cases.

Multiple-ribbon-lead components are highly suitable for

high density printed board applications due to their close

lead spacing, 1.25 mm, and small body sizes. The wide

variety of flatpack configurations permits a wide variety of

mounting methods.

The basic flatpack mounting technique can be categorized

as being either the through-the-board mounting or surface

mounting. These can be further divided into:

Through-the-board, unclinched lead.

Through-the-board, clinched with circumscribing full

land.

Through-the-board, clinched with offset land.

Surface land.

Surface post.

A number of discrete devices (transistors and diodes) are

also available as ribbon leaded devices. Most of these have

been designed specifically for high frequency, stripline

mounting.

11.1 Part Type Descriptions

11.1.1 Flatpacks

One of the smallest of the presently

existing multiple lead component type is the flatpack. The

body of these components can be as small as 3.2 mm wide,

6.4

mm long, and 0.8 mm thick. The component leads are

normally flat ribbons

0.5

x 0.25 mm or smaller, and are

located on 1.27 mm centers. Flatpacks are presently avail-

able with up to

50

leads. Various approaches are used to

facilitate the securing of these to carriers:

Form leads to fit a staggered hole-land pattern, using pot

or wave soldering.

Weld or solder leads to lands or tabs.

Form leads with stress relief bends to protect the glass

seals, then weld or solder to lands or tabs that are 0.8 mm

wide x 1.9 mm long. In an alternate method, all leads are

simultaneously solder coated and then reflow-soldered

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services